Process for making pulp from fibrous material



Jan. 9, 1934- o. KREISSLER PROCESS FOR MAKING PULP FROM FIBROUS MATERIALFiled Sept. 30,

1952 2 Sheets-Sheet. l

Jan. 9, 1934.

PROCESS o. KREISSLER FOR MAKING PULP'FROM FIBROUS MATERIAL Filed Sept.50, 1932 2 Sheets-Sheet. 2

Fig. 3-

(O/mm,

Q Patented Jan. 9, 1934.

UNITED STATES PROCESS FOR MAKING PULP FROM FIBROUS MATERIAL om mailer,FrankIort-on-the-Main, Germany, asaignor to American Lurgi Corporation,New York, N. Y., a corporation of New York Application September so,1932, Serial No.

635,673, and in Germany February 17, 1932 14 Claim.

This invention relates to a process for making 'pulp from fibrousmaterial.

In making wood pulp, especially with sulphite liquor, it has hithertobeen the custom to fill the digester charged with wood completely withthe liquor, steam being then admitted in order to obtain the requisitecooking temperature. The steam condensed, and the condensed waterincreased the volume of liquid in the digester. Moreover, gases,especially sulphur dioxide, were disengaged from the liquor during thecooking process. The rise of premure, through the increased volume ofliquor and the disengaged gases, attained the maximum working pressure(about 5 atmospheres)- for which the pulp digesters are usuallydesigned, in a comparatively short time (usually about 2-3 hours) afterthe commencement of the cooking process. In order to relieve thepressure, it was therefore necessary to open relief pipes in the upperpart of the digester from time to time. In the early stages of thecooking process, the chief portion of the discharge through these pipesconsists of liquor, which was either cooled and mixed with fresh liquor,or. passed to another digester without intermediate cooling with freshliquor. In the further course of the cooking process, when the level ofthe liquor in the digester had fallen, gases were mainly drawn off onthe relief pipes being opened, 30' these gases being utilized in thesame way as the relief liquor. For the most part, the discharged liquorand gases had to be stored beforeheing used again, because theoperations cannot be regulated so as to enable the relieved liquor and.

gases to .be utilized immediately in another digester.

This method of procedure is attended with a number of serious drawbacks.In the first place, liquor and gas are withdrawn from the cookingprocess and require special apparatus for their recovery andutilization. These quantities of liquor and gas also carry awayconsiderable quantities of heat from. the digester, thus causingcorresponding heat losses. In the di= gester itself, the, liquor wasraised to cooking temperature-prior to the expulsion oi the liquor andgases, and this heating chest was obtained.

by the condensation of corresponding amounts of steam, the portion ofthe latter required for heating the discharged liquor and gases losingunprofltably expended, for the most part, whilst the corresponding waterof condensation from that steam needlessly dilutedthe liquor in thedigester.

:3 Moreover, the existing method of working has (Cl. se-v) the drawbackof necessitating the use of liquors with a very high 80-. content, inorder to enable the liquor, progressively becoming weakened or dilutedthrough the continuous loss of gas during the cooking process, to remainsufficiently strong co to convert the wood into cellulose up to the endof the cooking process. It is well known, however, thatsuch initiallyhigh SO: content in the liquor has an unfavourable efiect onthe'character of the fibre, during the early stage oi cooking. 5 that is tosay, the fibres that are first converted into cellulose are alreadyattacked by the strong whereas the bulk of the fibre has: not yet beenconverted.

The composition of the liquor removed from the digester during theseveral stages of reliev ing and blowing ofi was not always the same. Atthe commencement of the cooking process it was substantially higher insulphur dioxide than during the further course of the process, theresult being to came special culties in reutilizing the expelled liquor.The ounts of liquor and gas drawn ofi from the digester bein veryconsiderable, it was not directly possible to blend the severalconcentrations accurately go and bring the liquor, freshly prepared withthe aid of the relief liquor, to the same standard for all cooks.Consequently, each succeeg cook was'afiected in a djifierent way, andprevented the attainment of a uniform qualityoi cellulose 5 throughout.Furthermore, the irregular composition of the liquor, resulting from therelic and the re-e nployment of the relieved gases and v liquids, alsohas an important influence on the length of the cooking process, therebycausing 9o irregular working loads throughout the entire series ofoperations, including the raising of steam, preparation of the liquorand wood material and so forth. Finally, the relieving of the gas andliquor is attended with special losses of material.

It has also been proposed to supply heat to the digester in a moreuniform manner, d to r the liquor in that vessel, the arm igeinentsadopted consisting of such propulsive devices as pumps or injectors.These serve to trmer liquor from a given point of the digester-the topfor one. pie-to the bottom or middle, or to one or more other places. Inthe case oi" these appliances, the liquor was also,"in some cases,heated up outside the digester, -on its way from one point of thedigester to another. Where this heating was not resorted to, thecirculat devices were also disposed inside the digester itself. Eventhese proposals, however, failed to remove the drawbacks arising fromthe increase of pressure during the cooking process'and, in particular,the repeated relieving of liquor and removal of gases, in order torelieve the pressure, were inevitable if the capacity of the digesterwas to be fully utilized, that is to say,the. charge of wood. in thedigester was not to be reduced.

The process of the present invention enables these defects to beovercome. It consists in that, although the digester is fully chargedwith wood at the outset, it is no longer completely filled with liquor,the amount of the latter being reduced approximately by the quantitynecessarily removed during the cooking process by the expulsion ofliquor and gas, in the method of working hitherto pursued. It has beenascertained that, by operating according to the process of the presentinvention, the quality of the cellulose does not sufier in any way but,on the contrary, is substantially improved inasmuch as, at thecommencement of cooking, such'portions of, the wood as are not coveredby the liquor are not subjected to any damage. As the cooking prosgresses, the wood sinks down to such an extent that the wholeis'submerged in the liquor, and, from that time onward any damage isnaturally impossible. p v 7 If howeven'it shouldbe desired to employspecial measurestogensure that the wood exposed at the commencement ofcooking is protected against damage, this protection can be obtained byimpregnating the exposed wood with liquor before cooking commences;

According to the present invention, the preferred method of penetratingwith liquor the wood that is exposed in the upper part of the digesterbefore cooking commences, consists in pumping the digester full ofliquor at the outset. It .is preferable to keep the pump running untilthe pressure in the digester has risen to about 3 atmospheres, or to adegree corresponding to the normal maximum working pressure of 5atmospheres or more. When this has been done, the, liquor in excess overthe limit according to the present invention, is drawn oil, for examplefrom the lower part of the digester without opening the top part of thedigester, after which the cooking is commenced. Drawing off the liquorfrom the lower part of'the digester, sets up a negative v pressure inthe upper part, and during the removal of the liquor, part of the aircontained in the chips of'wood in the top, escapes from the interior ofthe latter and is at once replaced by liquor. In consequence of thepartial vacuum, sumcient gas is liberated by the cooking liquor to allowthe liquor to be withdrawn to the extent prescribed by the invention,without any necessity for readmitting air into the upper part of thedigester. Finally, this complete filling of the digesterwith liquor andthen withdrawing a. portion of the latter effects the completeevacuation of the air in the upper part of thedigester.

The liquor to be withdrawn prior to cooking may, however, be taken fromany other part of the digester, such as at the top or the middle. In

such case the requisite pressure drop for the outflow of liquor from thedigester can be produced by means of a pump, or the like, or the vacuumset up in'the upper part of the digester through the withdrawal ofliquor may be wholly or partially nullified by admitting gases into thedigester, preference being naturally given to gases which are favourablefor the cooking process, such as sulphur dioxide, relief gases fromother 'peratures, such as 125 0.,

digesters, or roasting-furnace gases. Both measures may, of course, beapplied simultaneously, the liquor being withdrawn below and the gasesadmitted into the digester in such quantities as to set up a positivepressure, in the upper part, which facilitates the withdrawal of theliquor. It has been found in practice that, in the process according tothe invention, the pressure in the digester barely exceeds thepermissible pressure, throughout the cooking process, so that it ishardly necessary to draw off any of the liquor, or any substantialquantities of gases during the cooking process. At the most, even incooking with highly concentrated liquor, it is only necessary to removesmall quantities of gas during the last few hours, and, in consequence,nearly all the sulphur dioxide present at the outset of cooking isretained until the end. 1 g This advantage can also be utilized byemploying very weak initial liquors, such as tower liquors, for cooking.Practical experience has shown that, when weak digester can be run atvery low maximum temwithout any increase in the length of the operationand without having to draw off any gas during the cooking process. Thismethod furnishes a pulp of the best quality and purity, equal to thatotherwise obtainable only by indirect cooking, that is to say, in whichthe heat is indirectly supplied to the cooking liquor. In using strongliquor according to the present invention, it is unnecessary to draw offany gas until the main conversion of the wood into cellulose has beeneffected. For example, in the case of the known processes carried onwith the digester completely filled with liquor, the first withdrawal ofgas commencedat a temperature of 70-80 0., whereas, in applying theinvention, blowing off the gas-and that to a smaller extent-does notbecome necessary until the temperature has reached 130 C. and more.

According to the present invention, the circumstance that, owing to thereduced charge of liquor, larger quantities of gas can be stored in theupper part of the digester, is utilized to advantage by continuouslyreturning these gases to the lower part of the digester. At the sametime, this measure helps to lessen the increase of pressure in thedigester, because, under the increased liquid pressure in the lower partof theidigester, larger quantities of gas are cumstance also increasesthe strength 01 the liquor to a certain extent, thereby naturallyfacilitating the cooking process. A point that must be regarded as aparticular advantage of the invention is that the heat supplied to thedigester is solely expended in effecting the cooking process. It will beevident that, in consequence, and because smaller quantities of liquorhave to be heated, the heat consumption for each cooking is veryfavourable. Moreover, under such optimum conditions of the consumptionof heat and liquor, all the dig'esters in a mill can be arranged andoperated so that each works independently and the (strength of liquor,temperature and pressure) liquors are used, the

working conditions being always constant, it is now possible, for the Yfirst time, to obtain a product of uniform quality and quantity from allunits.

The return of the gases from the upper part of the digester into thecooking process can be effected by known devices, such as compressors,turbo-blowers or injectors. Such injectors can be operated, forinstance, by the steam which has to be employed for the cooking in anycase.

. If necessary, the'gas-return pipe can be conin the most favourablemanner.

nected to a pipe, coming irom outside and serving to introduce the othergases into the cooking process. The liquor itself can also becirculated. in known manner, either from below upwards, or vice versa,or vertically or laterally from the centre outwards,,and the return ofthe gases can also be arranged in the same manner. Of course, in allthese cases, there is nonecessity for the liquor circulation and thereturn of the gases to be combined.

If the circulation of the gases and liquor be combined in such a mannerthat the gases and liquor mingle at those points in the circulationwhere the maximum pressure prevails, the reabsorption-of the gases intothe liquor is eiiected In any event, the circulation of the gases andliquor in the process according to the invention also contributessubstantially to maintaining the concentration of sulphur dioxide in theliquor, up to the end of the cooking process, at a level hithertounattainable by employing initial liquors of the same concentration. Forexample, in practical working, where the initialliquor contained 4.1% ofsulphur dioxidefthe, S01: content of the end liquor still amounted to2%.

It follows therefore that, according to the invention, the amount of thedecomposing agent, at

the beginning oi the cooking process, is small by comparison with theconditions in the known processes, the liquor in action towards the endof the process contains substantially more of said agent than in theknown processes. This ac= counts for the fact that the celluloseobtained in accordance with the invention is of excellent quality, asthe result of its uniform treatment throughout theentire cookingprocess.

With the process according to the invention, relief gases are not formeduntil the cooking is finished and the positive pressure in the digesterhas been sumciently lowered to enable the contents of the digester to bedrained oil. In operating with high initial concentrations and cookingtemperatures above l25-l30 C., it is also occasionally necessary toremove relief gases during the final hours of the cooking process. Suchgases, however, can be easily utilized in the cooking plant, since theirvolume is smalland their composition always uniform.

They may be introduced, for example, into tower liquor, storage tanks,or into other digesters operating in the initial stage. In the lattercase, the process according to the invention affords the furtheradvantage that pressureincreasing conveying devices are notunconditionally required, since,'in operating in accordance with theinvention, the pressure in the several digesters increases unii'ormlyandgradually. Accordingly, in all cases, gases from a digester which hasbeen cooking for a long time can be allowed to flow directly intoanother digester' which has been started later. This was impracticablein the known processes, because the maximum permissible working pressurein each digester was already obtained in the early stage, for exampleafter cookinghad been in progress vfor 2-3 hours.

According to the invention, the removal of the relief gases can also beconsiderably accelerated by increasing, with the aid 01' known conveyingdevices, such as injectors, blowers or compressors, the inherentpositive pressure by which the gases are conveyed from the digester thatis to be relieved to the point of utilization. When the cooking isfinished, the digester can be made ready for the draining ofl of thecharge by admitting water or similar liquids and thereby completelydisplacing the gases present in the digester. In such case no alterationis needed in the conveying device or in the utilization of the gases.

By the process according to the invention, the

agents employedin the cooking process are therefore utilized to amaximum extent. In particular, with this maximum utilization of theliquor employed and the steam required for cooking, no.

special plant for the recovery oi heat and sulphur dioxide is requiredany longer.

In various casesit has already been the practice to fill the digesteronly partially with liquor, for instance with rotary digesters, or whenreturning the gases from the upper part of the digester to the lower. Inall such cases, however, the charge of wood in the digester has beenreduced in the same degree, and therefore the capacity of the digestercould not be utilized to the full. on the other hand, with the processaccording to the invention, the digester is completely filled with wood,and even mechanical filling devices can be employech They have,moreover, special advantages, since the saving in steam and sulphur, andalso the quality of the resulting pulp then attain a maximum.

In order more clearly to understand the nature of the present invention,reference will be made to the accompanying drawings, which illustratediagrammatically and by way of example, one embodiment of the inventionin which forated member '7. The-gases from the upper part of thedigester are returned to the cooking liquor by the injector 8, mountedin the delivery pipe 6 of the pump 2, and the intake pipe 9, terminatingin the strainer 10 high up in the digester. For the purpose ofconnecting the delivery pipe 6 and intake pipe 4 with the externallymounted pump 2, they are passed into a single conduit 11 provided in thewall of the digester and from which the delivery pipe 12 and intake pipe13, fitted with the throttles 14 and 15 respectively, lead to the pump2. A pipe 16, provided with the valve 17, for throttling the steam forthe cooking process, opens into the pipe 12.

The pipe 19, fitted .with the strainer 18 and throttle 20, serves fordrawing off liquor from the digester. The gauge 21, fitted with thecooks 22 and 23 serves for inspecting the height of liquor in thedigester. 24 is the inlet and 25 the outlet of the digester. Steam canbe introduced into the digester through the pipe 26, whilst the pipe 28can be employed for charging the digester with liquor. These pipes alsoare provided with throttles, 27 and 29 respectively. The pipe 30, withvalve 31, serves to admit water into the digester. Gases can be led outof the digester through the pipe 32, provided with valve 33.

In working with this apparatus, the process according to the inventionis carried out, for example, in the following manner:---

The digester 1, with a capacity, for example, of 225 cu. metres, ischarged with chips, either by gravity from the chip bin overhead, or bymeans of a mechanical filling apparatus, the outlet having first beenclosed. The valve 29 is then opened and liquor is admitted into thedigester through the pipe 28, the inlet 24 being left open. when thedigester has been completely filled with liquor, and the interstitialuntil a pressure of, for sample 3 atmospheres, or

'so that the gases and also the maximum permissible working pressure ofthe-digester, is attained. The valve 29 is then closed and the valve 20in the pipe 19 opened. Liquor is then allowed to draw ofi througbthepipe 19 to such an extent that no further withdrawal will be requiredduring the cooking proc-- eat, 26 cu. metres for example being run offin the case-of' a completely filled digester of 225 cu. metres capacity.The quantity of liquor removed can be accurately determined by watchingthe gauge glass 21 in which, after the cooks 22 and 23 have been opened,the liquorv attains the same level as in the digester. The digester topis kept shut while the liquor is being run on. Since pulp digesters, asa are sumciently tall, and gases are disengaged from the cooking liquorwhen negative pressure sets in, the vacuum produced in the upper part ofthe digester while the liquor is running off, cannot interrupt the Howof the latter.

When the level of the liquor in the digester has fallen to the desiredextent, the valves 20, 22 and 23 are closed, and cooking is commenced bystarting the liquor pump 2 and opening the valve 17 in the steam pipe16. If desired, the steam for cooking, or additional steam for thatpurpose, can be admitted into the digester by opening the valve 27 inthe pipe 28. The pump 2 draws the liquor in through the intake branches5 and pipe 4, from the upper part of the digester, and forces it throughthe pipe 6 and perforated head '7 into the bottom of the digester. Inaddition, the gases collecting in the upper part of the digester aredrawn of! through the pipe 9 and injector 8 and are mixed with thecirculating liquor in saidinJector and also in the pipe, being more orless completely absorbed by the liquor,

liquor enter the bottom of the digester together.

In the event of 'the'maximum working pressure of, for example, 5atmospheres, being attained in the digester before the end of thecooking process, the'valve 33 may be opened, to allow gas to escapethrough the pipe 32 and relieve the digester pressure, This pipe 32 alsoserves for carrying oif the gases after the cooking is finished. Duringthis latter relieving of the gases, water can be admitted into thedigester thi'ough the pipe 30. when the digester pressure has fallen toa sufflcient extent, the pulp is discharged through the outlet 25, andthe digester is temperature for a fresh charge.

Figure 3 shows the course of the to the invention. The

air in the chips dis-' placed, the inlet 24 is closed, but the pumpsupand pressure in a cooking operation aoxaording rises gradually andsteadily, and the maximum pressure is not attained until cooking hascontinued for 10 hours at a temperature of 130 C.,

so that it is not until the end of that period that the withdrawal ofgas will be required if the cooking temperature is to be furtherincreased.

I claim:

l. A process for making pulp from fibrous material with sulphite liquorwhich comprises charging the digester before cooking begins with aquantity of liquor which is smaller than usual by the amount that wouldotherwise have to be removed by relieving and blowing-oil, withoutreducing the quantity of wood charged into the digester, and irrigatingthe wood above the level of the liquor with cooking liquor and thereuponcarrying out the cooking in the usual manner.

2. A process for making pulp from fibrous material with sulphite liquorwhich comprises charging the digesterfbefore cooking begins with aquantity of liquor which is smaller than usual by the amount that wouldotherwise have to be removed byrelieving and blowing-oil, withoutreducing the quantity of wood charged into the digester, and irrigatingthe wood above the level of the liquor with cooking liquor at thecommencement of the cooking process.

3. A process for making pulp from fibrous material with sulphite liquorswhich comprises filling the digester full of both wood and cookingliquor before cooking commences, drawing off so much oi the liquor aswould otherwise have to be removed by relieving and blowing-on,thereupon commencing cooking and circulating the liquor and returningthagases from the upper part of the digester into the cooking liquor.

4. A process for making pulp from fibrous material with sulphite liquorswhich comprises filling the digester full oil both wood and cookingliquor before cooking commences, subjecting the liquor to pressure,drawing off so much of the liquor as would otherwise have to be removedby relieving andblowing-ofl, thereupon commencing cooking andcirculating the liquor and returning the gases from the upper part ofthe digester into the cooking liquor.

5'. A process for making pulp from fibrous maerial with sulphite liquorswhich comprises filling the digester full of both wood and cookingliquor before cooking commences, subjecting the liquor to a pressurewhich can be increased to the maximum permissible working pressure ofthe digester, drawing off so much of the liquor as would otherwise haveto be removed by relieving and blowing-oil, thereupon. commencingcooking and circulating the liquor and returning pressure p in thedigester the gases from the upper .part of the digester into the cookingliquor.

'6. A process for making pulp from fibrous material with sulphiteliquors. which comprises filling the digester full of both wood andcooking 'material with sulphite liquors which comprises filling thedigester full of both wood and weak cooking liquor beforecookingcommences, drawlog off so-much of the liquor as would otherwisehave to be removed by relieving and blowingofi', thereupon commencingcooking and circulating the liquor and returning the gases from theupper part of the digester into the cooking liquor.

8. A process for making pulp from fibrous material with sulphite liquorswhich comprises filling the digester full of both wood and cookingliquor containing 2 to 4% sulphur dioxide before cooking commences,drawing ofl so much of the liquor as would otherwise have to be removedby relieving and blowing-off, thereupon commencing cooking andcirculating the liquor and returning the gases from the upper part ofthe digester into the cooking liquor.

9. A process for making pulp from fibrous material with sulphite liquorswhich comprises filling the digester full of both wood and weak digesterliquor, drawing ofi so much of the liquor as would otherwise have to beremoved by relieving and blowing-off, thereupon commencing cooking at alow maximum cooking temperature and circulating the liquid withoutcirculating the gases.

10. A process for making pulp from fibrous material with sulphiteliquors which comprises filling the digester full of both wood andstrong cooking liquor, drawing on so much of the liquor as wouldotherwise have to be removed by relieving and blowing-off, thereuponcommencing cooking and circulating the liquor and only returning thegases from the upper part of the digester into the cooking liquor whenthe temperafure reaches 120-130 C.

11. Process of making pulp by cooking fibrous material with sulphiteliquor, distinguished by the fact that the cooker is filled'coinpletelywith the material to be cooked, before the cooking, but

the cooker.

is filled with liquor only so far that the material to be cooked is atfirst not completely immersed therein, as long at the beginning of thecooking as the cooking temperature exhibits a low degree of heatpreventing discoloration of the cooking material above the level ofimmersion, complete immersion of the material in the liquor occurringonly upon increase of the cooking temperature in connection with therise in the level of the liquor produced by the boiling.

12. Process according to claim 11, distinguished by the fact that thecooker first is filled entirely before the cooking operation both withthe material to be cooked and with the liquor, and then only is thereduced liquor filling produced before the cooking begins.

13. Process according to claim 11, distinguished by the fact that thecooker first is filled entirely before the cooking operation both withthe material to be cooked and with the liquor, the latter being underpressure, andthen only is the reduced liquor filling produced withproduction of a partial vacuum in the upper cooker chamber filled withmaterial to be cooked, before the cooking begins.

14. Process according to claim 11, distinguished by the returning ofexpelled gases and of the liquor from the upper part into the lower partof thecookingchamber by a pump inwhich both the gas and the liquor linesare arranged inside the cooker and the two are connected together by ajet apparatus fed by the liquor pipe, whereby the gases are drawn oiifrom the space above the liquor level and moved with the aid of the jetapparatus fed by the liquor pipe inside OTTO EISSLER.

